I am interested in trail riding 50 miles or less from Somis, CA. 
Where are the equestrian trails in Ventura County CA?

Here are parks with equestrain hills, many are within your distance
requirements. http://www.lamountains.com/parks_activities_equestrian.html

Also, there is a website just for horse topics in Ventura County. 
There is a chatbord on trails:
http://www.horseplaycentral.com/forumtest/forum/forum_topics.asp?FID=3.

Have fun!! Some of these places look beautiful.

Hi!  There is a really great trailhead to the Santa Monica mountains,
the back of Boney Ridbge, located in Thousand Oaks (Wendy off ramp?). 
From Somis, you can also take Huenemee (spelling) road east and the
trail head is on your right.  Ample trailer parking and
equestrian-only trails all the way (barring rainouts) to the pacific
ocean.  Make sure your horse is prepared for a little bridge work!
Join Equestrian Trails Internation (ETI) to join a trail group in your
area.  Best!  Lisa